Emissions from an engine system may be controlled with a catalyst coupled to an engine exhaust system. In order to maintain high catalyst efficiency, air-fuel ratio of the exhaust gas passing through the catalyst needs to be closely regulated. Air-fuel ratio of the exhaust gas may be controlled via controllers by adjusting a fuel injection amount using a mix of feedforward and feedback control loops. Tuning the controllers under various engine operation conditions may be complicated and time consuming. The complexity arises from a lack of understanding of the engine system and difficulty of isolating the underlying cause of the varied system response.
Other attempts to determine control parameters include tuning the controller through relay feedback. One example approach is shown by Boiko et al. in U.S. Pat. No. 8,255,066B2. Therein, oscillations corresponding to a selected gain or phase margin are generated, and PID controller tuning parameters are computed based on the amplitude and frequency of the oscillations.
However, the inventors herein have recognized that an identification that is specifically aimed at the appropriate model, in this case an automotive exhaust after-treatment system, versus a generic controller adjustment, provides more insight and coverage of varied operating conditions. A simple model that is just adequate to capture the dynamic response of the system in the frequency range of interest may resolve the controller tuning issue. The model may be easily characterized and can be incorporated into the controller structure. Further, control response may benefit from an update on-line to the original (in factory) calibration of control parameters to address control parameter drift due to catalyst degradation over time.
In one example, the issues described above may be addressed by a method including during steady state engine operation, adjusting fuel injection to a cylinder responsive to sensor feedback from downstream of a catalyst volume based on control parameters, the control parameters determined based on system identification at a point of feedback control instability. In this way, during engine operation, control parameters may be updated online with minor impact on engine/catalyst operation. Further, the updated control parameters may better account for system degradation and preserve high catalyst efficiency.
As one example, air-fuel ratio upstream of a catalyst may be controlled via an inner feedback loop, and air-fuel ratio downstream of the catalyst may be controlled via an outer feedback loop. Control parameters of the outer feedback loop may be tuned off-line at each of a set of pre-determined mass flow rates upstream of the catalyst. The calibrated control parameters may be saved in an engine controller and used during engine operation responsive to engine operating conditions. The lookup table may be updated online during steady state engine operation. Specifically, an oscillation in air-fuel ratio downstream of the catalyst may be induced by controlling the inner feedback loop via a relay function. As such, the outer feedback control loop reaches feedback control instability, and control parameters may be updated based on system identification. In this way, control parameters may be updated online based on a minimalist dynamic characterization of the catalyst control loop with minor impact on engine/catalyst operation. The updated control parameters enable high catalyst efficiency being achieved under a wide range of engine operating conditions. Further, the lookup table may be generated off-line to provide an initial characterization for all operating conditions under controlled laboratory conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.